Staff Publications

Ultrasonic borehole image tools are generally considered to be of limited value as an interpretative sedimentological tool because thick packages of a single lithology have similar acoustic properties resulting in resolution of very little diagnostic detail. In addition, build-up of mudcake in porous sandstones typically masks geological information by impeding the acoustic signal. In contrast, in heterolithic sediments, differences in acoustic impedance between intercalated lithologies enable clear distinction of sandstones and mudrocks from ultrasonic amplitude images. Whilst resolution of internal detail is still limited, valuable diagnostic data may be obtained for the sandstones by careful examination of the geometries and orientations of their lower and upper contacts with enclosing mudrocks. This is the case in the Lower Kimmeridge Clay Formation reservoir in the Magnus Field, UKCS, using the Schlumberger Ultrasonic Borehole Imager tool (UBI). Here, the bulk of the reserves are held in relatively thin, depositionally flat-lying, high-density turbidite sandbodies which are donors to variably inclined and complexly bifurcating sandstone injections. Recognition of sandstone injections and an understanding of their orientations is crucial to future development of this reservoir because they may be the principal means of communication between sandbodies and also have pay potential. Whilst this is not possible using conventional openhole logs it is easily achieved using UBI amplitude images.